1990 Mercedes Benz SL500 PROBLEMS WITH STARTING THE CAR

I SOMETIMES PLACE THE KEY IN THE IGNITION AND TURN IT TO START THE CAR AND IT WILL NOT START. It appears to be dead. There is no sound. The dash board light are not on. The head lights will work, the doom light, even the radio. I keeep tring to put the key in and out and turn it over. I have a cut over shitch in the trunk near the battery, which I turn on and off. After several attempts it will start up like it was new. I have after market stero and a clifford alrm, rdar etc. So there is a constanct drain on the battery. Thi one is 3 years old and is good. I have gone through 3 in 10 years. I just do not know if it is the ignition, starter, starter relasy or some sort of electrial short? Please let me know what you think it could be and what I should do to repair it?